Several methods of resistivity logging while drilling have been developed in the past. A method using toroidal coils around the drill stem mandrel to act as low frequency current transmitters and receivers (Arps U.S. Pat. No. 3,305,771) is currently employed and provides resistivity logs similar to the wireline laterolog tools. The high frequency wave propagation method (Gouilloud et al U.S. Pat. No. 3,551,797) provides logs similar to the wireline induction tools, and is currently employed widely in the industry. Recently the wave propagation method was improved by the incorporation of the borehole compensation feature and the dual depth of investigation feature (Clark et al U.S. Pat. No. 4,968,940). Both of these recent improvements, however, had been previously disclosed by Calvert (U.S. Pat. No. 3,849,721) and Huchital (U.S. Pat. No. 4,209,741) respectively, and the Clark disclosure builds on the combination of the previous ideas.
It has been standard practice in wireline resistivity logging tools to provide three measurements with different radial penetration, this being the minimum number required to solve quantitatively even the simplest model of formation invasion. While these methods are highly successful, the wireline data are gathered usually days or weeks after important reservoir rocks have been drilled, and can only observe the possibly damaging effects of drilling at excessive borehole pressure, resulting in severe invasion and possible plugging of porous formations by water or other borehole fluids. This may at the least require costly remedial actions, or, at the worst, result in an oil bearing formation being misinterpreted as water laden by the drilling fluid filtrate.
There is clearly a great value in observing invasion quantitatively while drilling (or immediately thereafter) so that the drilling engineer can adjust mud weights optimally. Similarly, the correction of resistivity data for invasion effects allows more accurate calculation of the hydrocarbon content of rocks which is of great importance in estimating reservoir potential. This can best be achieved as soon as possible after drilling. Analysis of the trend of resistivity data in otherwise uniform rock formations has been widely used as a predictor of over pressure mud conditions, so this information may also be used by the drilling engineer to adjust mud weights.
While it has not previously been possible to in situ measure a time profile of invasion during and immediately after drilling, such information could be of immense importance to reservoir engineers in determining the fluid permeability of rock formations in the virgin form encountered by the drill bit. Estimates of this data are obtained by wireline formation tester logging tools, but often are inaccurate due to the difficulty of mechanically displacing fluids in a localized area of the borehole wall and of overcoming the effects of previous formation invasion.
Finally, it is sometimes difficult for wireline logging tools to acquire information due to mechanical difficulties after drilling through poorly consolidated formations or in smaller borehole diameters. It is possible that a well may have to be abandoned due to the inability to use wireline tools or to equipment failure. Clearly, if a measurement while drilling could be made that provides data of equal quality to the wireline tool (accuracy, thin bed resolution, invasion corrected, etc.), then it would result in significant operational and economic benefits. This might avoid delay to run wire line tools.
One of the advantages of the present invention derives from the fact that three different depths of measurement are incorporated in an MWD tool. In the preferred and illustrated embodiment, a drill collar is constructed having an axial passage for delivery of mud How through the lower end of the collar and it is preferably located just above the drill bit. Indeed, it comprises the lower part of several drill collars typically found in a drill string. Moreover, it supports coils on the exterior used to form fields in the adjacent formations to make measurements. Measurements are made at three depths which are generally a shallow measurement which is obtained at a relatively low frequency, and higher frequency coils are used to make investigations at intermediate and maximum depths of investigation. This arrangement of the equipment enables resistivity to be determined at three depths which is significant for obtaining data with regard to the rate of filtrate invasion into the formations.
Consider as an example a well where the drill bit is momentarily totally within a non-producing formation which is substantially impervious to the penetration of filtrate. As the drill bit passes through the lower interface of that formation and enters a producing sand formation, there is a consequential flow of filtrate out of the drilling fluid into that formation. Assuming that a pressure differential does prevail and some portion of drilling fluid will enter the formation, the filtrate will displace the connate fluids. In ordinary circumstances, one can presume that the formation is axisymetric about the well borehole and the filtrate will therefore flow radially outwardly in an equal omnidirectional fashion. The filtrate rate of flow measured radially from the borehole requires typically several hours, and indeed several days, to reach the distance from the borehole at which the deepest area of investigation occurs. The sensors supported on the drill collar are positioned so that measurements are made at the three depths, and provide this information regarding invasion in the midst of the filtrate invasion thereby providing data showing the virgin formation material measurements.